The objective of this study is to determine the role played by light-induced free radicals in chemically induced skin photosensitivity. Light irradiation of 4-aminobenzenesulfonamide (sulfanilamide) resulted in the production of the following radicals: 4-H2NO2SC6H5, SO2NH2 4-NH2C6H5SO2, SO3-. Under the same conditions 4-aminobenzoic acid yielded 4-HOOCC6H5 and H free radical. Musk ambrette (4-t-butyl-3-methoxy-2, 6-dinitrotoluene) gave rise to a mixture of two nitroanion free radicals upon irradiation in methanol. The generation of these free radical species may explain the phototoxic and photoallergic properties of these compounds.